SEC-viscometry results

TABLE 4, SEC/VISCOMETRY RESULTS FOR THREE BROAD MWD PMMA SAMPLES... 

TABLE 5. SEC/VISCOMETRY RESULTS FOR A POLYVINYL CHLORIDE SAMPLE... 

TABLE 7. SEC/VISCOMETRY RESULTS OF STAR PS-W (WHOLE POLYMER)... 

TABLE 8. SEC/VISCOMETRy RESULTS OF TWO POLYVINYL ACETATE SAMPLES... 

Polystyrene. Table 3 shows the results obtained for three broad MWD polystyrene samples. The agreement for M and M values obtained from SEC/Viscometry analysis and the nominaY values supplied by the vendors is excellent. In addition the Mark-Houwink parameters, K and a values also are in excellent agreement with each other as well as with literature values.(28,39) This shows the consistency of the analysis method and the technique for determining the dead volume between detectors. The lower M value for the NBS 706 sample is due to the low molecular weight tail associated with the sample. 

Polymethyl Methacrylate. Table 4 shows the results obtained for three commercially available polyraethyl methacrylate samples. Again the molecular weight averages obtained with SEC/Viscometry are in good agreement with the nominal values and the K and o values are self-consistent and in excellent agreement with the literature values.(40)... 

Figure 8. Molecular weight distribution of branched PVAc (B...) as determined by laser light scattering (LLS) and size exclusion chromatography (SEC)/viscometry. A comparison of results by the two methods is listed in Table 2.

The computer models described provide a functional simulation of SEC-viscometry-LS analysis of linear polymers. The results for the Flory-Schulz MWD are in qualitative agreement with previous results for the Wesslau MWD. Both models emphasize the importance of determining the correct volume offset between the concentration detector and molecular weight-sensitive detectors. For the Flory-Schulz model, the peak shape, as well as the peak elution volume, can provide information about molecular weight polydispersity. Future work will extend the model to incorporate peak skew and polymer branching. 

The molecular weight of PVAc (the precursor to PVA) can be reliably measured in tetrahydrofuran (THF) using SEC-viscometry (33). The use of differential viscometry and LALLS provides a means to examine long-chain branching in the hydrolysis of PVAc to PVA. It is known that this hydrolysis results in the loss of hydrolyzable, long-chain branches that extend from the ester to the main PVAc backbone (10). Of course, branches formed by hydrogen abstraction and chain transfer to a carbon atom in the PVAc backbone are nonhydrolyzable and remain. 

The experimental techniques for the study of conformational branched properties in solution are the same as used for linear chains. These are, in particular, static and dynamic light scattering, small angle X-ray (SAXS) and small angle neutron (SANS) scattering methods, and common capillary viscometry. These methods are supported by osmotic pressure measurements and, nowadays extensively applied, size exclusion chromatography (SEC) in on-line combination with several detectors. These measurements result in a list of molecular parameters which are given in Table 1. 

Calculation of Results from Differential Viscometry and SEC. The familiar relationship first expressed by Mark (25) and Houwink (26) in the 1940 s is central to the concept of universal calibration first suggested by Benoit et al. (27). In this relationship,... 

The tetra- and hexafunctional initiators shown in Scheme 30 have been used for the polymerization of styrene and acrylates. The resulting stars had low PDIs, and the molecular weights obtained by SEC equipped with the lALLS detector and viscometry showed a good correlation with the theoretical values. In addition, a 6-armed star-block copolymer composed of a PMMA core and poly (isobomyl acrylate) shell was synthesized in this work. 

Modem SEC instruments in R D are often equipped with three or four detectors in series. However, this does not minimize the required user input and knowledge of the single methods applied (SEC, light scattering, and viscometry) is indispensable. Additional system parameters, such as the inter-detector delay, need to be determined carefully as they influence the results [13]. In addition, band broadening due to use of several detectors can be a problem [14-16]. 

While extensive research has led to a number of elegant ways to determine accurate MMDs for homopolymers using SEC, the characterization of copolymers is still notoriously difficult. For homogeneous copolymers, that is, those with extremely narrow CCDs, combinations of SEC with viscometry and or light scattering may be used. In fact, such copolymers with a constant chemical composition may be treated very much like homopolymers. However, for copolymers that exhibit both an MMD and a CCD no easy solutions exist. Two-dimensional separation techniques are required and comprehensive two-dimensional liquid chromatography (LC x SEC see Section 7.4.4) is developing into a routinely applicable technique. However, cahbration of LC x SEC so as to obtain accurate results on (co)polymer distributions is far from trivial (Table 7.3). 

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